Post Memory Corruption Memory Analysis Jonathan Brossard CEO – Toucan Systemjonathan@toucan-system.com
Who am I ? (a bit of self promotion ;)- Security Research Engineer, CEO @ Toucan System (French Company).- Known online as endrazine (irc, twitter...)- Met some of you on irc.- Currently lives in Sydney (pentester for CBA).- Speaker at several conferences : Defcon/HITB/HES/Ruxcon/h2hc...- Organiser of the Hackito Ergo Sum conference (Paris).
Agenda A few basics Being environment aware PMCMA Design Extending Pmcma •Stack desynchronization
Whats pmcma ?Its a debugger, for Linux (maybe one day *NIX) ptrace() based.Pmcma allows to find and test exploitation scenarios.Pmcmas output is a roadmap to exploitation, not exploit code.Tells you if a given bug triggering an invalid memory access is a vulnerability, if it is exploitable with the state of the art, and how to exploit it.
How do applications crash ?* Stack corruptions -> stack overflows, usually now detected because of SSP | studied a LOT* Signal 6 -> assert(),abort(): unexpected execution paths (assert() in particular), heap corruptions* Segfault (Signal 11) -> Invalid memory access
Invalid memory access- trying to read a page not readable. often not mapped at all.- trying to write to a page not writable. often not mapped at all.- trying to execute a page not executable. often not mapped at all.
Why do they happen ?Because of any kind of miscomputation, really :- integer overflows in loop counters or destination registers when copying/initializing data, casting errors when extending registers or- uninitialised memory, dangling pointers- variable misuse- heap overflows (when inadvertently overwriting a function ptr)- missing format strings- overflows in heap, .data, .bss, or any other writable section (including shared libraries).- stack overflows when no stack cookies are present...
Exploiting invalid exec Trivial, really. Eg : call eax with eax fully user controled
Exploiting invalid memory reads ?- usually plain not exploitable- wont allow us to modify the memory of the mapping directly- in theory : we could perform a user controled read, to trigger a second (better) bug.
How to...To exploit invalid writes, we need to find ways to transform an arbitray write into an arbitrary exec. The most obvious targets are function pointers.
Exploiting invalid memory writes : scenario- Target a known function pointer (typically : .dtors, GOT entry...).Can be prevented at compile time : no .dtors, static GOT...- Target function pointers in the whole binary ?- Overwrite a given location to trigger an other bug (eg : stack overflow)
GOALS- We want to test overwriting different memory locations inside a process and see if they have an influence over the flow of execution- We want to scale to big applications (web browsers, network deamons...)- We want a decent execution time
mk_fork() The idea :-We start analysing after a SEGFAULT-We make the process fork() (many many times)-Inside each offspring, we overwrite a different memory location
mk_fork() : benefitsMapping looks « just like » it will when actually exploiting a binary No ASLR/mapping replication problem Exhaustive and hopefully fast
How to force a process to fork ?1) Find a +X location mapped in memory.2) Save registers3) Use ptrace() to inject fork() shellcode.4) Modify registers so eip points to shellcode.5) Execute shellcode.6) Wait() for both original process and offspring.7) Restore bytes in both processes.8) Restore registers in both processes.
mk_fork() Offspring n Executable Writable Executable …
mk_fork() : PROS- allows for multiple tests out of a single process- fast, efficient (no recording of memory snapshots)- no need to use breakpoints- no single stepping
mk_fork() : CONS- Dealing with offsprings termination ? (Zombie processes)- I/O, IPC, network sockets will be in unpredictable state- Hence syscalls will get wrong too (!!)
Zombie reaping- Avoid the wait() for a SIGCHILD in the parent process.- Kill processes after a given timeout, including all of their children.
Zombie reaping : the SIGCHILD problemIf we can have the parent process ignore SIGCHILD signals, we wont create Zombies.=> We inject a small shellcode to perform this via sigaction()
Zombie reaping : the SIGCHILD problem1) Find a +X location mapped in memory.2) Save registers3) Use ptrace() to inject sigaction() shellcode.4) Modify registers so eip points to shellcode.5) Execute shellcode.6) Wait() for the process while executing shellcode.7) Restore bytes in +X location.8) Restore registers in the process.
Zombie reaping : killingthe offsprings and their childrenFortunatly, this is possible using « process grouping »...
Process groupingsetpgid() sets the PGID of the process specified by pid to pgid. Ifpid is zero, then the process ID of the calling process is used. Ifpgid is zero, then the PGID of the process specified by pid is made thesame as its process ID. If setpgid() is used to move a process fromone process group to another (as is done by some shells when creatingpipelines), both process groups must be part of the same session (seesetsid(2) and credentials(7)). In this case, the pgid specifies anexisting process group to be joined and the session ID of that groupmust match the session ID of the joining process.
Zombie reaping : forcing process grouping1) Find a +X location mapped in memory.2) Save registers3) Use ptrace() to inject setpgid() shellcode.4) Modify registers so eip points to shellcode.5) Execute shellcode.6) Wait() for the process while executing shellcode.7) Restore bytes in +X location.8) Restore registers in the process.
Force process grouping...;; setpgid(0,0); shellcode;_start: nop nop nop nop mov eax,0x39 ; setpgid xor ebx,ebx xor ecx,ecx int 0x80 db 0xcc, 0xcc
Zombie reaping : final detailsFrom now on, to kill a process and all of its children : kill (-pid, SIGTERM) ;
IPC, I/O, invalid syscallsOne possibility is to recode correct execution on the original process (after clearing signals and ignoring the SEGFAULT).Then replay/fake the syscalls on the offsprings.=> Minimal userland « virtualization ».
Exploiting invalid memorywrites via function pointersWe now want to find all the functionpointers called by the application from the instruction which triggered the SEGFAULT until it actually halts.(including pointers in shared libraries!!)
Finding all the function pointers actually called1) Parse all the +W memory, look for possible pointers to any section1 bis) optionally disassemble the destination and see if it is a proper prologue.2) use mk_fork() to create many children3) in each children, overwrite a different possible function pointer with a canari value (0xf1f2f3f4).4) Monitor execution of the offsprings
Finding all the function pointers actually calledOverwritten pointer leads to execution of canari address 0xf1f2f3f4 <=> We found a called function pointer.
Finding all the functionpointers actually called DEMO
So what can we test now ? Invalid write anything anywhere : attacker has full control over datawritten and destination where written => GAME OVER
So what can we test now ?Overflows (in any writtable section but the stack) : Simply limit the results of pmcma to this section.
So what can we test now ?What if the attacker has little or no control over the data beingwritten (arbitrary write non controled data, anywhere) ?
Partial overwrites and pointers truncationIf we cant properly overwrite a function pointer, maybe we can still truncate one (with the data we dont control) so that it transfers execution to a controled memory zone ?
Exemple :--[ Function pointers exploitable by truncation with 0x41424344:At 0xb70ce070 : 0xb70c63c2 will become 0xb70c4142 (lower truncated by 16 bits, dest perms:RW)At 0xb70e40a4 : 0xb70ca8f2 will become 0xb70c4142 (lower truncated by 16 bits, dest perms:RW)At 0xb70ec080 : 0xb70e5e02 will become 0xb70e4142 (lower truncated by 16 bits, dest perms:RW)At 0xb731a030 : 0xb7315da2 will become 0xb7314142 (lower truncated by 16 bits, dest perms:RW)At 0xb73230a4 : 0xb732003a will become 0xb7324142 (lower truncated by 16 bits, dest perms:RW)At 0xb732803c : 0xb7325a36 will become 0xb7324142 (lower truncated by 16 bits, dest perms:RW)At 0xb76a80d8 : 0xb7325bf0 will become 0xb7324142 (lower truncated by 16 bits, dest perms:RW)
One more situation...Sometimes, an attacker has limited control over the destination of the write (wether he controls the data being written or not). Eg : 4b aligned memory writes.
Exploiting 4b aligned memory writesWe cant attack a function pointerdirectly, unless it is unaligned (rare because of compiler internals).Pmcma will still let you know if this happens ;)
Exploiting 4b alignedmemory writes : plan BFind all « normal » variables we can overwrite/truncate, and attempt totrigger a second bug because of this overwrite.
Finding all unaligned memory accesses Setting the unaligned flag in theEFLAGS register will trigger a signal 7 uppon next access of unaligned memory (read/write).
Finding all unaligned memory accesses DEMO x86_64
Defeating ASLR : Automated memory mapping leakageHow does WTFuzz did it at CansecWest 2010 to win the pwn2own contest against IE8/Windows 7 ? Overwrite the null terminator of a JS string to perform a mem leak uppon usage (trailing bytes).
Defeating ASLR with an arbitrary write ?In the original process :- use ptrace() PTRACE_SYSCALL- record the calls to sys_write() and sys_socketall() (wrapper to sys_send() or sys_sendto()...), including : where is the data sent ? How many bytes ?
Defeating ASLR with an arbitrary write ?Create many offsprings using mk_fork().-In each of them : overwrite a different location with dummy data.-Follow execution using PTRACE_SYSCALL-Monitor differences : a different address or a bigger size means a memory leak :)
Means of modifying theflow of execution without function pointers Call tables. Calling [Offset+register] => This is also already performed automatically using pmcma.
Pointers and ASLRIf overwritting a given function pointer isnt practical because of ASLR : is it possible to overwrite a pointer (in an other section) to a structure containing this function pointer ? Would this « other section » be less randomised ?
Finding pointers to structures containing function pointers Executable Writable (no ASLR) Executable Writable (high Complex structure ASLR) … Executable void* f(a,b,c) …
Finding pointers to structures containing function pointers Wed like to have the debugged process create a new section, with a given mapping (to ease identify). Modify a possible pointer per offspring (use mk_fork()). Monitor execution : is the offspring calling a function pointer from our custom mapping ?
Forcing a process to create a new mapping :1) Find a +X location mapped in memory.2) Save registers3) Use ptrace() to inject mmap() shellcode.4) Modify registers so eip points to shellcode.5) Execute shellcode.6) Wait() for the process while executing shellcode.7) Restore bytes in +X location.8) Restore registers in the process.
Testing exhaustively arbitrary writesIn case all of the above failed...Can we trigger secondary bugs by overwritting specific memory locations ?
Testing exhaustively arbitrary writes Complexity is huge ! Still doable with Pmcma, with noguaranty over the time of execution.
Testing exhaustively arbitrary readsIn the same veine, attacker controled invalid reads can trigger secondary bugs, which will be exploitable.=> We can test the whole 4+ billions search space (under x86 Intel architecture), or just a few evenly chosen ones.
Stack desynchronization W^X is a problem. Even if we can overwrite fully afunction pointer and modify the flow of execution... what do we want to execute in 2011 ?
Stack desynchronizationInstead of returning directly to shellcode in +W section (hence probably not +X) :-Return to a function epilogue chosen so that esp will be set to user controled data in the stack.- Fake stack frames in the stack itself.- Use your favorite ROP/ret2plt shellcode
Stack desynchronization : Exemple : sudo- stack is ~1000 big (at analysis time)- we find a function pointer to overwrite (at 0x0806700c)- we overwrite it with a carefully chosen prologue (inc esp by more than 1000)
Stack desynchronization : Exemple : sudojonathan@blackbox:~$ objdump -Mintel -d /usr/bin/sudo... 805277a: 81 c4 20 20 00 00 add esp,0x2020 8052780: 5b pop ebx 8052781: 5e pop esi 8052782: 5d pop ebp 8052783: c3 ret
Stack desynchronization : Exemple : sudoWe can control the destination where esp is going to point : simply use an environment variable TOTO=mydata sudo
Stack desynchronization : Exemple : sudoWe then forge fake stack frames in the stack itself- « Nop sled » : any pointer to retEg :804997b: c3 ret- Then copy shellcode to .bss byte per byte using memcpy via ret2plt- Use GOT overwrite to get pointer to mprotect() in the GOT (ROP)- call mprotect to make .bss +X via ret2plt- return to shellcode in .bss